Retinoids are defined as substances that can elicit specific biological responses by binding to and activating a specific receptor or set of receptors. Retinoids are known to play a fundamental role in normal cell growth and differentiation. (Roberts, A. B. et al, in "The Retinoids," ed. by M. B. Sporn, A. B. Roberts and D. S. Goodman, Vol. 2, pp. 209-256, Academic Press, Orlando, Fla., (1984); Sporn, M. B. et al, J. Amer. Acad. Dermatol., 15:756-764 (1986)). Multiple retinoic acid nuclear receptors. (RAR.alpha., .beta. and .gamma.) and retinoid X receptors (RXR.alpha., .beta. and .gamma.) have been identified (Evans, R. M., Science, 240:889-895 (1988); O'Malley, B. W., Mol. Endocrin., 4:363-364 (1990); Gudas, L. J. Cell Growth Differ, 3:655-662 (1992)); Lohnes et al, Cell Sci., 16 (Suppl): 69-76 (1992). Moreover, numerous isoforms of the various nuclear receptors exist as a result of alternative splicing (Gudas L. J., J. Biol. Chem., 269:15399-15402 (1994)).
Retinoic acid receptors mediate gene transcription through a variety of mechanisms. These nuclear receptors can bind to specific DNA consensus sequences termed retinoid receptor response elements (RAREs or RXREs) which are located in the regulatory regions of the retinoid target genes (Gudas, L. J., Cell Growth Differ., 3:655-662 (1992); Lohnes et al, Cell Sci., 16 (Suppl.):69-76 (1992)). Nuclear receptor binding to these response elements preferably occurs through heterodimer formation between the RAR and RXR, although homodimer binding and subsequent gene activation has also been found (Hermann et al, Mol. Endocrinol., 6:1153-1162 (1992); Leid et al, Cell, 68:377-395 (1992); Zhang X, Nature, 355:441-446 (1992)). The RXRs can mediate gene transcription via heterodimer formation with the RARs, with the vitamin D, thyroid hormone (Yu et al, Cell, 67:1251-1266 (1991); Hermann et al, Mol. Endocrinol., 6:1153-1162 (1992); Kliewer et al, Nature, 355:446-449 (1992); Leid et al, Cell, 68:377-395 (1992); Zhang et al, Nature, 355:441-446 (1992)), and a number of orphan receptors. (Apfel et al, Mol. Cell Biol., 14:7025-7035 (1994); Song et al, Proc. Natl. Acad. Sci., USA 91:10809-10813 (1994)). These orphan receptors can, in turn, inhibit the activity of RARs and thyroid nuclear receptors (TRs) (Kliewer et al, Proc. Natl. Acad. Sci., USA, 89:1448-1452 (1992); Tran et al, Mol. Cell Biol., 12:4666-4676 (1992); Apfel et al, Mol. Cell Biol., 14:7025-7035 (1994), Casanova et al, Mol. Cell Biol., 14:5756-5765 (1991); Song et al, Proc. Natl. Acad. Sci., USA, 91:10809-10813 (1994)).
The retinoid receptor response elements usually consist of direct repeats (DRs) in which the half-sites are separated by a number of base pair spacers. Selectivity for binding appears to be determined by the number of base pairs utilized as spacers, as well as by the sequence of the response element itself (Kim et al, Mol. Endocrinol., 6:1489-1501 (1992), Mader et al, J. Biol. Chem., 268:591-600 (1993)).
RAR and RXR inhibition of AP-1-mediated gene transcription that does not require RAR or RXR binding to DNA has also been observed (Pfahl, Endocrin. Reviews, 14:651-658 (1993), and references cited therein); RAR and RXR when completed to their ligands have been shown to inhibit c-Jun/c-Fos binding to the AP-1 consensus sequence and subsequent gene activation (Pfahl, Endocrine Reviews, 14:651-658 (1993), and references cited therein). Negative regulation of transcription by RA can apparently also occur by means that do not involve RAR binding to the promoter region but by inhibiting enhancer activity (Gudas, J. Biol. Chem., 269:15399-15402 (1994)). In addition, negative regulation of RAR-mediated, as well as TR-mediated, gene transcription occurs by the competitive binding of the orphan receptor coup and v-ErbA to RARE and TREs (Tran et al, Mol. Cell. Biol., 12:4666-4676 (1992), Hermann et al, Oncogene, 8:55-65 (1993)).
Most cell types express more than one RAR and RXR receptor. RAR homologous recombination studies have suggested that RAR functional redundancy exists among the different RARs (Li et al, Proc. Natl. Acad. Sci., USA, 90:1590-1594 (1993), Lohnes et al, Cell, 73: 643-658 (1993), Lufkin et al, Proc. Natl. Acad. Sci., USA, 90:7225-7229 (1993)). However, other studies have indicated that the various receptor subtypes possess distinct functions and may indeed modulate the activity of distinct genes (Nagpal et al, Cell, 70:1007-1019 (1992); Boylan et al, Mol. Cell Biol., 15:843-851 (1995)). Evidence also suggests a unique role for each of the receptor subtypes: (1) receptor selectivity towards specific transactivating response elements has been demonstrated (Nagpal et al, Cell, 70:1007-1019 (1992)); and (2) specific cell types have become refractory to the antiproliferative and differentiating effects of RA with the loss of one receptor subtype, despite the presence of other RAR subtypes (Sheikh et al, J. Cell Biochem., 53:393-403 (1993); Moasser et al, Oncogene, 9:833-840 (1994)).
The RARs bind both RA and its isomer 9-cis-RA, while the RXRs only bind 9-cis-RA (Allenby et al, J. Biol. Chem., 269:16689-16695 (1995), and references cited therein). To further document a unique function for each receptor subtype, conformationally restricted retinoids have been synthesized that selectively bind to and enhance transcriptional activation by selective RAR and RXR subtypes (Graupner et al, Biochem. Biophys. Res. Commun., 179:1554-1561 (1991); Lehmann et al, Cancer Res., 61:4804-4809 (1991), Lehmann et al, Science, 258:1944-1946 (1992); Dawson et al, in "Retinoids: New Treatments in Research and Clinical Applications". Livrea M A and Packer L., (eds) Marcel Dekker: NY pp 205-221 (1992); Davies et al, Amer. Ass'n of Cancer Res. Conf., Banff, Alberta, Canada, Mar. 15-20 (1993) Abst. B-28; Jong et al, J. Med. Chem., 36:2605-2613 (1993); Reichert et al, from "Mol. Biol. to Therapeutics: Pharmacology of the Skin", Vol. 5, Bernard B A and Shroot B (eds), Karger: B.2d pp 117-127 (1993); Beard et al, Bioorg. Med. Chemical, 4:1447-1452 (1994); and Boehm et al, J. Med. Chem., 37:2936-2941 (1994)).
These synthetic receptor-selective retinoids have further confirmed the uniqueness of specific RAR sub-types in modulating RA responses in various cell types (Rudd et al, Cancer Letter, 73:41-49 (1993); Sheikh et al, J. Biol. Chem., 269:21440-21447 (1994)). Recently, a series of synthetic retinoids has been described that selectively transactivate RAR.gamma. (Bernard et al, Biochem. Biophys. Res. Comm., 186(2):977-983 (1992)).
Because of the ability of retinoids to affect cell growth and differentiation, these compounds have been disclosed to be useful for the treatment or prevention of diseases and conditions involving abnormal cell proliferation and differentiation. For example, the usage of retinoids as efficient therapeutics for the treatment of various skin diseases and neoplasms has been reported (Roberts, A. B. and Sporn, M. B., in "The Retinoids", Sporn et al, pp 209-286, Academic Press, Orlando, Fla; Bollag et al, Ann. Oncol., 3:513-526 (1992); Smith et al, J. Clin. Oncol., 10:839-864 (1992)).
To date, the best results of retinoid therapy have typically been achieved with a regimen which combines retinoid administration with the administration of other differentiation or cytotoxic agents. Besides retinol and retinoic acid, isotretoin (13-cis-retinoic acid) and etretinate have been used, as well as 9-cis retinoic acid and N-(9-hydroxyphenyl)retinoid.
The most convincing results have been documented in the field of dermological disorders, where topical application can circumvent the toxic effects sometimes observed during systemic administration of retinoids. For example, retinoids have been reported to be useful for the treatment of a variety of dermatoses including psoriasis, cystic acne, cutaneous disorders of keratinazation, among others.
Besides dermatological disorders, retinoids have important potential as anti-cancer agents. For example, retinoid compounds have been disclosed to have potential for the prevention of skin cancer, for the treatment of acute myeloid leukemia (AML), acute promyelocytic leukemia (APL) for the treatment of other hematopoietic malignancies such as myelodysplastic syndrome, juvenile chronic myelogenous leukemia, Sezary syndrome, squamous cell carcinomas of the upper aerodigestive tract, non-small lung cancer, and human head and neck carcinomas. (See Pfahl et al, Vitamins and Hormones, 49:327-382 (1993) at 363-366, which reviews the usage of retinoids as therapeutics).
In the specific case of breast cancer, the growth of some breast cancer cell lines has been reported to be inhibited by retinoids (La Croix et al, J. Clin. Invest., 65:586-591 (1980)). Also, etretinate has been reported to prevent the growth of xenotransplanted breast carcinoma cells in athymic mice (Halter et al, Cancer Res., 48:3733-3736 (1988)). Further, it has been reported that N-(4-hydroxyphenyl) retinamide (4-HPR) induces apoptosis and the differentiation of breast cancer cell lines, assertedly independent of the status of estrogen receptor (ER) and RAR expression (Pellegrini et al, Cell Growth Differ., 6(7):863-869 (1995)).
Also, a recent patent issued to Curley et al, U.S. Pat. No. 5,516,792, assigned to Ohio State Research Foundation, teaches the use of retinoyl beta-glucuronide N-glycoside analogs for the treatment, prevention and study of cancers, including breast cancer. Further, N-(4-hydroxyphenyl) retinamide (4-HPR), a derivative of trans-retinoic acid, is currently in clinical trials as a chemopreventive agent for breast cancer.
Additionally, retinoic acid in combination with RAR-.beta. receptors has been reported to promote apoptosis of estrogen receptor-positive (ER+) human breast cancer cell lines (Liu et al, Mol. Cell Biol. 16(3):1138-1149 (1996)).
Further, the use of retinoic acid in combination with interferon, specifically alpha interferon or gamma interferon, has been reported to inhibit the proliferation of some breast cancer cell lines (Widschwendter et al, Anticancer Res., 16 (1):369-374 (1996); Widschwendter et al, Cancer Res., 55(10):2135-2139 (1995)).
Also, 9-cis retinoic acid has been reported to inhibit the growth of breast cancer cells and to down-regulate estrogen receptor RNA and protein (Rubin et al, Cancer Res., 54(24):6549-6556 (1994)).
Still further, the use of (4-HPR) in combination with the anti-estrogen tamoxifen as a potential synergistic combination for breast cancer chemoprevention has been reported (Costa, A., Eur. J. Cancer, 29A(4):589-592 (1993)).
However, while some retinoids have been reported to have potential as anticancer agents, and specifically for the treatment or prevention of breast cancer and leukemia, the identification of retinoids having improved therapeutic properties which are suitable for the treatment or prevention of such cancers would be highly beneficial. In particular, the identification of retinoids which are cytotoxic to either estrogen receptor positive or estrogen receptor negative breast cancers would be highly beneficial.